This invention relates to an apparatus and method for bond force control, and in particular to such control in a die bonding operation.
In a die bonding operation in a semiconductor assembly and manufacturing process, a bond head may pick a die (which may be an electronic device such as a semiconducting integrated circuit device) at a first location and will move the die to a second location at which the die is to be bonded to a substrate, such as a circuit board. The bond head will then move downwardly (this is known as the z-direction) until the die touches the substrate. In order to apply a required bond force during the bond operation, the bond head may be driven downwardly further to exert a required force on the die.
Apparatus and methods for controlling this bond force need to meet a number of conflicting requirements. For example, they should preferably be able to apply a bond force over a wide range, should be susceptible to feedback and control, and should be able to respond rapidly to required changes in the bond force.
To date prior art techniques have failed to satisfactorily meet all the requirements for ideal bond force control. Conventionally bond force control is achieved using a compression spring, or a pneumatic actuator, or a solenoid, and a number of examples of such conventional prior art bond force control techniques will now be shown.
FIG. 1 shows a compression spring type system. In such a system a compression spring 1 is mounted within the bond head 3 and exerts a force on a moving assembly 2 that connects to a collet bearing the die. In a die bonding process a die will be held (for example by vacuum) to a collet formed as part of the moving assembly 2. The bond head is moved downwardly in the z-direction until the die touches the substrate. In order to exert a bonding force the bond head 3 is continued to move in the z-direction so as to compress the spring 1 against the moving assembly 2 and to exert a bond force on the die through the collet.
In order to avoid excess vibrations of the moving assembly 2 during motion of the bond head 3, the spring 1 is usually preloaded and the amount of preload directly affects the minimum bond force that can be applied. The larger the preload, the larger is the minimum bond force that can be applied. This is because when the collet and die touch the substrate (which is a rigid surface), the reaction force that pushes the collet and die against the spring should be larger than the preload of the spring. This means that if a very small bond force is required, the preload of the spring must also be very small, but this in turn has the disadvantage of resulting in more vibrations of the bond head following movement which will thus require a longer settling time in the bonding operation.
Another problem that arises from simply reducing the preload of the spring to enable a small bonding force to be applied, is that the contact of the spring with the moving assembly 2 and the bond head housing 3 may be poor and the bond force may be non-linear. This can be mitigated by reducing the spring constant k so that a larger extent of compression, good contact, and small preload can all be obtained at the same time. The price for this, however, is that the range of the output force produced by the spring is reduced and this method can only really work with small bond force applications.
U.S. Pat. No. 5,950,903 discloses a system similar to the use of a compression spring, but using a leaf spring instead of a compression spring.
FIG. 2 shows a schematic of a prior art example that uses a pneumatic actuator instead of a compression spring. A pressure chamber 4 is provided within the bond head housing 6 and acting upon the moving collet assembly 5. In this arrangement a variable bond force can be applied by controlling the pressure within the pressure chamber 4. A difficulty with this design, however, is that the response time is slow. If the pressure in the chamber 4 is controlled using a pressure regulator the response time is too slow for a real time closed loop control system and instead an open loop control is used instead. A related disadvantage is that there is a relatively large impact force. At the moment that the die touches the substrate surface an impact force is produced that acts upon the die. If this impact force is too large it can damage the die, but the slow response time of a pneumatic system makes it very difficult to control the impact force. It is also difficult to control the bond force with any great accuracy.
In solenoid systems such as those described for example in U.S. Pat. Nos. 3,727,822, 4,603,802, 5,230458 and 5,696,329 a solenoid is used in place of a compression spring or a pneumatic actuator to provide the bond force. Solenoid systems have a number of advantages, including fast response times, the ability to incorporate closed-loop control, and easy accurate control. The main disadvantage, however, is that the output bond force is relatively small, and to create a large bond force when such is required, a large and correspondingly heavy solenoid must be used. Generally, solenoid systems are only suitable for use in applications where a small bond force is required.
It is an object of the invention to provide an apparatus and method for providing bond force control that facilitates the generation of a bond force over a wide range of values, allows fast response and closed loop control, and is simple in design and construction.
According to the present invention therefore there is provided apparatus for providing bond force in a die bonding operation, comprising means for generating a primary bond force, means for generating a compensatory bond force, and means for controlling the compensatory bond force whereby a bond force comprising the sum of the primary bond force and the compensatory bond force may be controlled. The compensatory bond force may be directed either in the same direction as the primary bond force or in the opposite direction to the primary bond force.
The primary bond force generating means may comprise a preloaded compression spring (other possibilities include a leaf spring or a pneumatic actuator). The compensatory bond force generating means preferably comprises a motor.
Preferably a force sensor is provided for sensing the applied bond force, and for producing an output for controlling the compensatory bond force generating means.
The primary bond force generating means may be movable relative to a die holding means whereby the bond force component provided by the primary bond force generating means may be varied by moving the primary bond force generating means relative to said die holding means (i.e. by driving in).
Viewed from another broad aspect the present invention provides a method of generating a desired bond force in a die bonding operation, comprising: providing a primary bond force, generating a compensatory bond force in the same direction or opposite to said primary bond force, summing said primary bond force and said compensatory bond force to generate said bond force, and controlling the magnitude and sign of said compensatory bond force to control said generated bond force.
Preferably the method further comprises sensing an instantaneous bond force, and controlling the compensatory bond force in response to the sensed instantaneous bond force. The compensatory bond force is preferably generated by a motor comprising a motor coil, and the magnitude and sign of the compensatory force may be controlled by varying the magnitude and direction of a current applied to the coil.
The primary bond force may comprise a preload component, and the generated bond force comprises the sum of said preload component and the compensatory bond force. The primary bond force may also include a variable component, and the generated bond force may then comprise the sum of the preload component, the variable component and said compensatory bond force.
The present invention also provides a method of controlling the impact force in a die bonding operation, comprising moving a bond head bearing a die toward a substrate in a first direction, and providing means for generating a compensatory force in a direction opposite to said first direction at the moment of impact of said die on said substrate.
The present invention still further provides a method of preventing vibrations of a collet assembly in a die bond head during movement of the said bond head either in the z direction prior to engaging the substrate or translational movement, comprising generating a force to move said collet assembly against a stop means while said bond head is being moved.